We have been examining polymorphisms in genes involved in the leptin signaling pathway to identify gene variants impacting on body composition. We are currently intensively studying a variant MC3R that is associated with adiposity in children and which appears to have functional significance for MC3R signal transduction. Children who were homozygous variant for both the polymorphisms (Thr6Lys and Val81Ile) had significantly greater fat mass and leptin compared with wild type or heterozygous children. In vitro studies subsequently found that expression was significantly lower for the double-mutant MC3R. Ongoing studies attempt to understand the mechanisms by which these sequence alterations may impact body weight. We have initiated a new study comparing energy balance during adaptation to a high-fat diet in humans with double-mutant and wild type MC3R. Data from subcutaneous abdominal adipose tissue biopsies suggest greater fatty acid uptake in humans with the double-mutant receptor. We have also successfully bred knock-in mice expressing the human wild type and human double-mutant MC3R. These mice have marked obesity but with surprisingly well-maintained adiponectin concentrations, a finding we have also identified in humans with the double-mutant MC3R. Additional work seeks to understand the structure of the MC3R transcript. We have identified previously-unknown starts of translation for both murine and human MC3R and elucidated the 3 structure (1). We have also investigated the BDNF-TrkB pathway in regards to body mass in children. We found among patients with the WAGR (Wilms Tumor, Aniridia, Genitourinary, and Renal abnormalities) syndrome that those missing one BDNF allele (BDNF+/-) had significantly greater body mass than BDNF +/+ during childhood. A full characterization of the energy intake and expenditure of subjects with WAGR syndrome and other 11p deletion syndromes is continuing. Interestingly, we were unable to show that BDNF serum concentrations were altered in humans with MC4R function altering mutations. We have found that leptin is an important predictor of weight gain in children: those with high leptin gain even more weight when followed longitudinally and identified children with hyperleptinemia and leptin receptor mutations (2). We have also found hyperleptinemia out of proportion with body fat mass in children with psychological loss of control over eating (3). Such data suggest the importance of leptin resistance as a factor stimulating weight gain and have led to recent explorations of other syndromes associated with obesity that may cause dysregulation of leptin signaling, including Bardet Biedl syndrome. Other studies are directed at understanding additional genetic, physiological, psychological, and metabolic factors that place children at-risk for undue weight gain (4-9). We have recently examined how obesity and puberty are interrelated (10,11) Two recent initiatives target insulin resistance. One trial studies the role of depressive symptoms in childrens insulin resistance. We have found both cross-sectional and prospective longitudinal associations between depressive symptoms and the development of insulin resistance and links between depressive symptoms and energy intake in children and adolescents. A randomized clinical trial to study the effects on insulin resistance of preventing depression in obese adolescents with a family history of type 2 diabetes is ongoing. Another pilot study initiated in 2013 examines how sedentary behaviors contribute to poor glucose tolerance and tests if short bouts of activity may improve glucose tolerance in non-overweight and overweight children. Investigations concentrating on binge eating behaviors in children suggest that such behaviors are also associated with adiposity in children, predict future weight gain in children at-risk for overweight, and predict both greater energy consumption during meals and decreased satiety after eating. The ability to consume large quantities of palatable foods, especially when coupled with decreased subsequent satiety, may play a role in the greater weight gain found in binge eating children. Using our longitudinal cohort, we have also found that the development of some aspects of metabolic syndrome in children is predicted by the presence of binge eating behaviors (10) although in adults most findings can be explained by increased body weight (19). Recent studies have identified alexithymia as increased in adolescents with loss of control over their eating (5), characterized the frequent weight loss efforts of such children, and found links between parents and childrens eating behaviors. These data also suggest that interventions targeting disordered eating behaviors may potentially be useful in preventing excessive fat gain in children prone to obesity. Two ongoing protocols examine efficacy of interpersonal therapy as a weight gain preventive strategy among children and adolescents who report binge eating behaviors (16). Given the rapid increase in the prevalence of obesity, the development of treatments for obesity is urgently needed. In clinical protocols, we have studied pharmacotherapeutic approaches to the control of body weight (17,18). Two placebo-controlled randomized trials studied the effects of medications on weight loss and on obesity-related comorbidities in children and adolescents. Orlistat 120 mg TID was studied in 200 adolescents, 61% African American, mean BMI 41.70.6 kg/m2. Adolescents treated with orlistat lost more weight (orlistat -2.90.7 vs. placebo -0.60.7 kg, P=0.011), BMI units (-1.720.24 vs. -0.700.24 kg/m2, P=0.002), and fat mass (-3.90.8 vs. -1.40.8 kg, P=0.029), but had little impact on obesity-related co-morbid conditions in overweight adolescents. Metformin 1000 mg BID was studied in 100 severely overweight children (6-12y) who manifested hyperinsulinemia and insulin resistance. Subjects participated in a monthly weight reduction program. Compared to placebo-treated children, those randomized to metformin decreased BMI (metformin -0.910.3 vs. placebo +0.230.3 kg/m2, P=0.006), BMI-Z score (-0.110.02 vs. -0.040.02, P=0.02), and body fat mass (-1.40.7 vs. +2.10.7 kg, P<0.001) to a significantly greater extent. Serum glucose (-2.40.9 vs. +1.61.2 mg/dL, P=0.018), HOMA-IR (-0.190.4 vs. +0.950.4, P=0.05), and total cholesterol (-9.92.7 vs. +1.14.8, P=0.04) also decreased more in metformin-treated compared to placebo-treated children. We concluded that metformin, added to a monthly behavioral program, significantly improved weight loss, insulin resistance, and cholesterol over a 6-month interval in severely overweight, insulin-resistant children. We will report on a 3-year open-label extension protocol to examine the long-term impact of metformin therapy on weight. We recently initiated a new collaboration to enable a multi-site randomized-controlled trial of beloranib, a methionyl aminopeptidase 2 inhibitor, to treat the hyperphagia of patients with the Prader Willi Syndrome. We expect to initiate additional translational trials in the near future related to modulation of the leptin signaling pathway.